With rapid advance of manufacturing technology, the requirement for devices of precision measurement is increasing, especially for those used for manufacturing precision products, such as micro-electro-mechanical system (MEMS) parts, integrated circuit (IC) wafer, liquid crystal display (LCD) panel, and so on. Recently, the use of interferometry for detecting 3-D surface contour of an object has been adopted by industries for improving production yield. Operationally, the interference pattern containing information corresponding to the surface profile of a measured object for reconstructing the surface profile of the object. For precisely reconstructing the three-dimensional surface profile of the object, a clear interference pattern should be used in the following reconstruction processes. It is noted that for interferometry, the closer the intensity of its reference light is to that of its object light, the higher the fringe contrast in the resulting interferogram will be. Therefore it can create clearer and sharper interference pattern.
Please refer to FIG. 1, which is a schematic diagram showing a conventional Mirau interferometer. By the Mirau interferometer in FIG. 1, an incident light 10 after being focused by an objective 110 is incident on a beam-splitting module 111 fitted in an optic module 11, at which the focused incident light 10 is split into an inspection light 103 and a reference light 104 in a non-polarization manner by a beam-splitting layer 113 in the beam-splitting module 111 while enabling the inspection light to illuminate on a measured object 90 where it is reflected thereby to form an object light 105 containing information corresponding to the surface profile of the measured object 90. At the same time, the reference light is first reflected back to the beam-splitting layer 113 by a reflector 112, and then the reflected reference light is combined with the object light by the beam-splitting module to form a combined light that passes through the objective 110. It is noted that the reference light 104 and the object light 105 in the combined light can interfere with each other to form an interference pattern.
Please refer to FIG. 2, which shows a first polarized component and a second polarized component in a combined light, where the polarizations of the two polarized component are orthogonal to each other. It is noted that for the incident light, reference light, inspection light and object light, they are all composed of the aforesaid first and second polarized components with orthogonal polarizations. Thus, the interference pattern in the combined light is composed of a first polarized interference pattern and a second polarized interference pattern, in which the first polarized interference pattern is created by the interference between the first polarized component of the reference light 104 and the first polarized component of the object light 105; and similarly, the second polarized interference pattern is introduced by the interference between the second polarized component of the reference light 104 and the second polarized component of the object light 105. In a conventional Mirau interferometer, the incident light 10 is not polarized, so the first polarized component and the second polarized component of the reference light 104 will have the same amplitudes and phases and that is also true for the object light 105. Therefore the first polarized interference pattern and the second polarized interference pattern are exactly the same while the superposition of the first and second polarized interference patterns in the combined light will only make the brightness of the superposed interference pattern created by the combined light double and won't make its fringe distribution change.
However, it is noted that there are some measured objects that have the ability to absorb or scatter much of the object light projected thereon, and consequently induce a big intensity difference between the object light 105 and the reference light 104. Therefore, the two polarized interference patterns, respectively created by the interference between the first polarized components of the object light 105 and the reference light 104, and the interference between the second polarized components of the object light 105 and the reference light 104, will have very low contrasts, so that the contrast of the interference pattern of the combined light is too low for analyses. Since there is no way in the conventional interferometers to adjust the relative intensities of the reference light and the object light in the combined light, the contrast of the superposed interference pattern can not be adjusted.
Therefore, it is in need of an method for adjusting the relative intensities of the reference light and the object light in the combined light so as to increase the contrast of the superposed interference pattern. For the above-mentioned reason, the reference light and the object light should be combined in an orthogonal-polarization manner before their interference to make the two lights not interfere with each other since the polarizations of the two lights are independent of each other. Because of the polarizations of the reference light and the object light are independent of each other before interference, modulating the amplitudes of the two lights respectively to reduce intensity difference between the two lights is possible. After modulating the polarizations of the two lights by using an analyzer, the polarizations of enabling the two lights are not orthogonal to each other and the two lights can interfere with each other, and consequently creating an interference pattern with high contrast. Moreover, it is also in need of a beam-splitting module and interference system adopting the aforesaid orthogonal-polarization Mirau interferometry that are capable of overcoming the problem of low-contrast interference pattern in the prior art.
There is already a study relating to polarization Mirau interferometry, which is an interferometric profilometer sensor disclosed in U.S. Pat. No. 5,166,751. The sensor defines a Mirau-like interferometer arrangement with a measured object surface and a reference surface. For precisely measuring a small distance change, a phase retarder is arranged to change the path difference between an object light and a reference light with two polarizations. Nevertheless, there are some differences between the method provided in this U.S. patent and an interferometric method provided in the present invention, which are:
(1) In the aforesaid U.S. patent, the incident light is split into an inspection light and a reference light in a non-polarization manner. However, it is intended in the present invention to split the incident light into an inspection light and a reference light with orthogonal polarizations.
(2) The main difference between the Mirau-like interferometer in the U.S. patent and those conventional Mirau interferometers is the additional phase retarder arranged in the Mirau-like interferometer. However, for meeting requirement of the phase retarder, the Mirau-like interferometer uses only narrow-band light. Nevertheless, the difference between the orthogonal-polarization interferometry in the present invention and those conventional Mirau interferometers is that: instead of the non-polarization manner in the conventional Mirau interferometers, the splitting of the incident light and the combination of the reference light and object light are accomplished in a orthogonal-polarization manner, so that the method in the present invention is adapted for the use of broadband light.
(3) In the aforesaid U.S. Patent, the reference light and the object light will interfere with each directly in the combined light. However, in the method provided in the present invention, the reference light and the object light can not interfere with each directly in the combined light and can only do so after each is being processed by a polarization modulation process.